This invention generally relates to arrays of micromachined sensors, including but not limited to micromachined ultrasonic transducers (MUTs). One specific application for MUTs is in medical diagnostic ultrasound imaging systems. Another specific example is for non-destructive evaluation (NDE) of materials, such as castings, forgings, or pipelines. However, the curved sensor arrays disclosed herein are not limited to ultrasound transducers, but rather may also comprise, e.g., temperature, pressure, or optical sensors.
Conventional ultrasound imaging systems comprise an array of ultrasonic transducers that are used to transmit an ultrasound beam and then receive the reflected beam from the object being studied. Such scanning comprises a series of measurements in which the focused ultrasonic wave is transmitted, the system switches to receive mode after a short time interval, and the reflected ultrasonic wave is received, beamformed and processed for display. Typically, transmission and reception are focused in the same direction during each measurement to acquire data from a series of points along an acoustic beam or scan line. The receiver is dynamically focused at a succession of ranges along the scan line as the reflected ultrasonic waves are received.
For ultrasound imaging, the array typically has a multiplicity of transducers arranged in one or more rows and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducers in a given row can be controlled to produce ultrasonic waves that combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused in a selected zone along the beam.
The same principles apply when the transducer probe is employed to receive the reflected sound in a receive mode. The voltages produced at the receiving transducers are summed so that the net signal is indicative of the ultrasound reflected from a single focal zone in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delay (and/or phase shifts) and gains to the signal from each receiving transducer. The time delays are adjusted with increasing depth of the returned signal to provide dynamic focusing on receive.
The quality or resolution of the image formed is partly a function of the number of transducers that respectively constitute the transmit and receive apertures of the transducer array. Accordingly, to achieve high image quality, a large number of transducers, referred to herein as elements, is desirable for both two- and three-dimensional imaging applications. The ultrasound elements are typically located in a hand-held transducer probe that is connected by a flexible cable to an electronics unit that processes the transducer signals and generates ultrasound images. The transducer probe may contain both ultrasound transmit circuitry and ultrasound receive circuitry.
Convex curved ultrasound probes are the dominant choice for abdominal imaging. They provide a large field of view with excellent resolution achieved with a relatively simple system beamformer. Most ultrasound probes are made from piezoelectric ceramics or single crystals. Each piezoelectric element vibrates in the thickness mode as a half-wave resonator. Engineers have managed to widen the bandwidth of these vibrators through an extensive collection of additional impedance matching layers, backings, and electrical tuning techniques.
Curved probes are generally fabricated by laminating piezoelectric ceramic, matching layers, and a backing together as a flat acoustic stack. The stack is subsequently diced (often from the back surface through the backing, ceramic, and part of the matching layers) at a very fine pitch to allow them to easily bend to conform to a curved shape. The resulting structure is then affixed to an appropriately shaped preformed backing block. A typical example is disclosed in U.S. Pat. No. 5,637,800.
It is well known in the art that any surface can be described at a point by two orthogonal radii of curvature, r1 and r2. The requirement that the surface remain continuous is equivalent to requiring the product of the inverses of the two radii of curvature to be a constant:
                              1                                    r              1                        ⁢                          r              2                                      =        constant                            (        1        )            Two radii of curvature of a flat plate are both infinite, and the constant is 0. Hence it is easy to curve the array in either the azimuthal or elevational direction, where one inverse radius remains 0. However, curvature in both directions is severely limited.
Recently semiconductor processes have been used to manufacture ultrasonic transducers of a type known as micromachined ultrasonic transducers (MUTs), which may be of the capacitive (cMUT) or piezoelectric (pMUT) variety. cMUTs are tiny diaphragm-like devices with electrodes that convert the sound vibration of a received ultrasound signal into a modulated capacitance. For transmission the capacitive charge is modulated to vibrate the diaphragm of the device and thereby transmit a sound wave. pMUTs are similar except that the diaphragm is bimorphic, consisting of a piezoelectric and an inert material like silicon nitride or silicon. The bimorphic diaphragm typically has greater sensitivity but lower bandwidth properties.
One advantage of MUTs is that they can be made using semiconductor fabrication processes, such as microfabrication processes grouped under the heading “micromachining”. As explained in U.S. Pat. No. 6,359,367:                Micromachining is the formation of microscopic structures using a combination or subset of (A) Patterning tools (generally lithography such as projection-aligners or wafer-steppers), and (B) Deposition tools such as PVD (physical vapor deposition), CVD (chemical vapor deposition), LPCVD (low-pressure chemical vapor deposition), PECVD (plasma chemical vapor deposition), and (C) Etching tools such as wet-chemical etching, plasma-etching, ion-milling, sputter-etching or laser-etching. Micromachining is typically performed on substrates or wafers made of silicon, glass, sapphire or ceramic. Such substrates or wafers are generally very flat and smooth and have lateral dimensions in inches. They are usually processed as groups in cassettes as they travel from process tool to process tool. Each substrate can advantageously (but not necessarily) incorporate numerous copies of the product. There are two generic types of micromachining . . . 1) Bulk micromachining wherein the wafer or substrate has large portions of its thickness sculptured, and 2) Surface micromachining wherein the sculpturing is generally limited to the surface, and particularly to thin deposited films on the surface. The micromachining definition used herein includes the use of conventional or known micromachinable materials including silicon, sapphire, glass materials of all types, polymers (such as polyimide), polysilicon, silicon nitride, silicon oxynitride, thin film metals such as aluminum alloys, copper alloys and tungsten, spin-on-glasses (SOGs), implantable or diffused dopants and grown films such as silicon oxides and nitrides.            The same definition of micromachining is adopted herein. The systems resulting from such micromachining processes are typically referred to as “micromachined electromechanical systems (MEMS).
Conventional cMUTs resemble tiny drums (made of silicon nitride or other similar materials, such as silicon) that are “beat” electrostatically. The drumhead vibrates to both emit and receive ultrasonic waves. A cMUT probe consists of an array of many elements, each element comprising a respective plurality of individual cMUT cells that have been hard-wired together.
A typical cMUT cell comprises a thin silicon or silicon nitride membrane with an electrode, suspended over a cavity formed on a silicon substrate. A bottom electrode is formed in or on the silicon substrate or by doping the substrate so that it is conductive. All cMUT cells in an element are electrically connected using top and bottom electrodes. The membrane vibrates to both emit and receive ultrasonic waves. The driving force for the deflection of the membrane during transmit is the electrostatic attraction between the top and bottom electrodes when a voltage is impressed across them. If an alternating voltage drives the membrane, significant ultrasound generation results. Conversely, if the membrane is biased appropriately and subjected to incoming ultrasonic waves, significant detection currents are generated. Typical thicknesses of the membrane lie in the range of 1 to 3 microns and the cavity gap is on the order of 0.1 to 0.3 micron. The lateral dimensions of the cMUT cell range from 10 to 100 microns for cMUT array operating frequencies of 2 to 15 MHz.
Capacitive micromachined ultrasound transducers are fabricated on flat crystalline silicon substrates, and represent a novel approach to ultrasound transduction. Since the micromachined membrane typically oscillates far above the operating frequency of the probe and is damped by the material (such as an acoustic lens) that is applied to the top surface, these devices are inherently broadband. Operating over a wider frequency range than the comparable piezoelectric transducers, these cMUTS should improve B-mode, color flow, and harmonic imaging. However, cMUT ultrasonic probes are usually built on very stiff, hard to bend substrates.
There is a need for methods of making curved cMUT (or PMUT) substrates without diminishing their acoustic performance advantages. In particular, there is a need for a method of making a convex curved cMUT substrate, thereby achieving the wide field of view so many clinicians find appealing in convex curved piezoelectric arrays used in abdominal imaging.